Well, think of it as going home. Say you are 2.5 miles away from your school, and you need to walk 2.5 more miles to your house, then say it's about to rain. Will you run back to school or to your home? The problem with this is it's close to each number because it's equal. If you add or subtract, you get both answers, so this is a trick question.
Every ordered pair has a second number of -6, so the mapping with -6 as the sole number on the right is the appropriate mapping. A mapping is only <em>not a function</em> if it maps a number to two or more different values (as shown in the top diagram of your attachment).
The appropriate choice is not shown in your question. It would be the one attached below.
